1. Field of the Invention and Related Art Statement
This invention relates to a heat-sensitive recording material which is superior in thermal response and forms little detritus which often sticks to a thermal head.
2. Discussion of Related Art
Generally, heat-sensitive recording materials comprise a support and, provided thereon, a heat-sensitive recording layer mainly composed of an electron donating normally colorless or a pale colored dye precursor and an electron accepting color developer which react with each other instantly upon being heated by a thermal head, a thermal pen or a laser beam to obtain recorded images. These are disclosed in Japanese Patent Kokoku Nos. 43-4160 and 45-14039. These heat-sensitive recording materials have the following advantages: Record can be obtained by relatively simple apparatus; maintenance is easy; and no noise appears. Thus, these recording materials are used in a wide variety of fields such as instrumentation, facsimiles, printers, terminals of computers, labels and tickets vending machines. Especially, heat-sensitive recording systems have been greatly demanded in the field of facsimile. Thus, there has been developed a speeding-up of recording for reduction of transmission cost and lowering of energy for attaining reduction of costs by miniaturization of facsimiles, and for the speeding-up and lowering of energy, enhancement of sensitivity of heat-sensitive recording materials has been demanded. On the other hand, hitherto, dot density of thermal heads has generally been 8 dots/mm, but now there is the demand, more than before, to print small letters in high image quality by increasing the dot density to 16 dots/mm, for example, and decreasing the dot area, to make gradational printing according to dither method and to obtain images of good print quality which are a faithful reproduction of the dot patterns of a head.
In order to meet these demands, when the recording sheet is subjected to strong supercalendering treatment to improve contact between the sheet and thermal head, so-called fogging occurs due to a reduction of the whiteness thereof.
Japanese Patent Kokai No. 56-27394 has proposed to provide an under coat layer between a heat-sensitive layer and a base paper according to which images of high density can be obtained with a low printing energy without strong supercalendering and thus enhancement of sensitivity has become possible. Application of this undercoat layer is effective for smoothing the surface of a heat-sensitive layer by filling-up irregularities on the surface of support to smooth the surface of the support.
In this way, enhancement of sensitivity has been made by application of an undercoat layer. However, application of an undercoat layer alone which aims at only smoothening of the surface cannot satisfy the recent demands for further enhancement of sensitivity and for improvement of the reproducibility of dot patterns.